Bottom Current Modification of Turbidite Lobe Complexes

Abstract

Submarine lobes form at the distal end of sediment gravity flow systems and are globally important sinks for sediment, anthropogenic pollutants and organic carbon, as well as forming hydrocarbon and CO2 reservoirs. Deep-marine, near bed or bottom currents can modify gravity flow pathways and sediment distribution by directly interacting with the flow or by modifying seafloor morphology. Deciphering the nature of gravity- and bottom currents interaction, particularly in ancient systems, remains a challenge due to the lack of integrated datasets and the necessary oceanographic framework. Here we analyse high-resolution 3D seismic reflection and core data from the Upper Cretaceous interval offshore Tanzania to reveal the interaction of turbidite lobes with fine-grained sediment waves and contourite drift deposits. Contourite drift morphology governs the large-scale confinement style and shape of lobes that range from frontally confined and crescent shaped, to laterally confined and elongated, to semi-confined lobes. Core data reveals massive to cross-laminated high density turbidites in the lobe axis position that show no direct interaction between gravity flows and contour currents. Lobe off-axis and fringe deposits consist of parallel- and ripple-laminated, low density turbidites, which are inter-bedded with bioturbated, muddy siltstones that represent the toes of contourite drifts. Starved ripples, and streaks of up to fine-grained sandstone above individual turbidite beds indicate reworking by bottom currents. This facies distribution reflects the temporal interaction of quasi-steady bottom currents and turbidity currents that interact with the topography and build lobes over short periods of time. Frontally confined turbidity currents form lobes in a fill-and-spill fashion, in which the confinement of turbidity currents causes rapid deposition and obscures any bottom current signal. Lateral confinement causes increased turbidity current runout length, and promotes the development of lobe fringes with a high proportion of bottom current reworked sands. During times when sediment gravity flows are subordinate, contourites accumulate on top of the lobe, confining the next flow and thus modifying the overall stacking pattern of the lobe complex. Although sediment volumes of these bottom current modified lobe complexes are comparable to other deep-marine systems, bottom currents considerably influence facies distribution and deposit architecture.

Sedimentary architecture of submarine lobes affected by bottom currents: Insights from the Rovuma Basin offshore East Africa

“…The Antarctic Circumpolar Current fully developed around the Eocene-Oligocene boundary due to the opening of the Drake Passage (Barker & Thomas, 2004;Livermore et al, 2004;Zachos et al, 2001). Therefore, during the Oligocene, the general circulation pattern along East Africa was probably similar to that observed today (Figure 1a; Fuhrmann et al, 2022;Thiéblemont et al, 2020). Bottom currents flowed northward along the East African margin leading to the development of extensive contourite drifts in the Mozambique Basin and in the Mozambique Channel, particularly during the Oligocene (Thiéblemont et al, 2020), and asymmetric channel-levee systems in the Rovuma basin, formed by the synchronous interaction of bottom currents and gravity flows (Fuhrmann et al, 2020(Fuhrmann et al, , 2022.…”

“…Bottom currents can strongly control sedimentation and erosion in several areas of the seafloor in deep-marine environments, and even affect turbidite systems, forming turbidite-contourite systems (Fonnesu et al, 2020;Fuhrmann et al, 2020;Miramontes et al, 2020). Since the intensity of bottom currents is strongly variable, their influence on seafloor morphology and sedimentation may also be different between systems (Fonnesu et al, 2020;Fuhrmann et al, 2020Fuhrmann et al, , 2022Miramontes et al, 2020Miramontes et al, , 2021Mutti et al, 2014). Moderate bottom currents are able to deflect the fine-grained sediment carried in suspension above the channels along slopes by turbidity currents (Miramontes et al, 2020).…”

“…However, these studies are limited to sedimentary systems that are purely controlled by gravity flows. In the presence of ocean currents, the morphology, scale, and stacking pattern of different hierarchical lobe architectures may be significantly modified (Fuhrmann et al., 2022). The influence of bottom currents (i.e., ocean currents flowing near the seafloor) on submarine‐fan architecture has been widely recognized, resulting in the formation of mixed turbidite‐contourite systems (hereafter mixed systems; Fuhrmann et al., 2020; Gong et al., 2018; Mulder et al., 2008; Rebesco et al., 2014).…”

“…Bottom currents mainly result in enhanced deposition in the levee located in the downstream direction of bottom currents and thus in the development of asymmetric channel‐levee systems (Fonnesu et al., 2020; Lu, 2017; Luan et al., 2021; Miramontes et al., 2020). In contrast, much less is known about the effect of bottom currents on the architecture of submarine lobes (Fonnesu et al., 2020; Fuhrmann et al., 2022). Previous studies have found that bottom currents can modify the distribution and architecture of the entire lobe system (Fuhrmann et al., 2022) or directly rework lobe deposits when bottom currents are intense (Mutti et al., 2014).…”

“…In contrast, much less is known about the effect of bottom currents on the architecture of submarine lobes (Fonnesu et al., 2020; Fuhrmann et al., 2022). Previous studies have found that bottom currents can modify the distribution and architecture of the entire lobe system (Fuhrmann et al., 2022) or directly rework lobe deposits when bottom currents are intense (Mutti et al., 2014). However, the effect of bottom currents on the different hierarchical lobe architectures in deep‐water environments is still not well understood.…”